Injection System for Small Diameter Capillary Columns

It is obvious that very small diameter capillary columns require very small samples. Samples used for capillary columns may amount to only a fraction of a microgram in size. It is clear that a sample of this size is too small for practical injection syringes and, as a consequence, a split flow system must be used. In practice, the sample is vaporized into a gas stream and a small portion of the gas stream (and consequently a small portion of the sample) is allowed to pass through the column. A diagram of a split flow injector is shown in figure 57.

Figure 57. The Split-Flow Injector

The split-flow injector is very similar to the packed column injector except that only part of the carrier flow passes to the column, the rest exits to waste. The split ratio can be adjusted over a wide range by varying the exit flow-impedance. The small-bore capillary columns would be virtually impossible to use without a split injection system of this type. However, because of the waste of sample that occurs with this type of injector, and because of the relatively small mass-range obtainable from small-bore capillary columns, the large bore capillary column was introduced.

Injection Systems for Large Bore Capillary Columns

Capillary columns that are sufficiently wide to permit a hypodermic needle to enter (viz. 0.052 in. I.D.) and, thus, allow on column injection are considered to be large-bore. A diagram of an injection system for large bore columns is shown in figure 58. The large bore capillary injector is also very similar to the packed column injector, however, in this case, the needle now penetrates into the open end of the large bore capillary column and can discharge its contents directly onto the column walls as opposed to the column packing. This type of injector, however, has a number of disadvantages. When the sample is injected into the column (either as a neat sample or a solution in a solvent), it breaks up into separate parts, as bubbles that form along the first part of the column..